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Fatigue treatment of wood by high-frequency cyclic loading S. Panula-Ontto1, M. Lucander1, T. Pöhler1, E. Saharinen1, T. Björkqvist2 1KCL 2Tampere University of Technology ABSTRACT New scientific tools for investigating the qualitative and quantitative response of wood to mechanical loading under conditions close to those of mechanical pulping are needed to find ways of radically reducing energy consumption. At KCL a modulated loading device was developed to fulfill these requirements. The device allows the static load, the loading frequency and the amplitude to which wood is subjected to be varied over a wide range. Wood samples were cyclically stressed at large strain (1 mm) pulses and at frequencies in the range 100-1000 Hz. The temperature rise in the wood was measured with thermocouples inserted into the wood and on the wood surface using a thermographic camera. It was found that a high static load during cyclic loading increased heat generation in the wood. The cyclic loading frequency had less effect than the level of the static loading. However, low frequencies generated more heat at a specific number of impacts. The responses of heartwood and sapwood were significantly different in the fatigue treatment. The temperature rise was higher in heartwood than in sapwood. The wood underwent irreversible plastic deformations seen as heat generation and increased pore volume in the wood microstructure. INTRODUCTION In theory wood can be converted mechanically to fibers suitable for printing paper production at specific energy consumptions considerably lower than those achieved today. To accomplish this, more fundamental knowledge of wood rheology and fatigue behavior is needed. The structure of any polymeric material can be weakened or broken down by mechanical action. It is well known that existing mechanical pulping processes are energy inefficient. One reason for this is the composite structure of wood, including the ingenious multilayer structure of the wood fibers. The aim of mechanical pulping is to weaken the wood structure, to separate the fibers from each other, and mechanically to knead the fibers into flexible and well-bonding particles without major loss of fiber length. To achieve all this, the mechanical pulping processes used today have to be relatively gentle, and are therefore highly inefficient. Although it is over 150 years since the first mechanical pulping processes were developed, theoretical calculations show that in today’s processes 80% or even more of the specific energy input is used just to generate heat and only some 20% to fatigue the fibers. Despite intensive research efforts since the 1960s only relatively small stepwise reductions in energy consumption have been achieved. The technical and economic breakthroughs have not been achieved. The viscoelastic properties of wood need to be taken into consideration in optimizing the defibration process towards lower specific energy consumption. The goal should be for most of the mechanical energy to be converted into permanent deformation and to a lesser extent into heat [1, 2, 3]. The mechanical behavior of a viscoelastic material change as temperature and deformation rate of the material vary. Viscoelasticity means that:
o the force needed to create a certain deformation increases with the strain rate. o the force needed to create a certain deformation decreases with increasing temperature. o stress-induced deformation is time dependent, i.e. there is a phase shift between applied load and arised
deformation in cyclic loading. When a polymeric viscoelastic material is stressed mechanically, some of the loading energy is converted into heat due to the viscous behavior. Heat generation can be used as an indicator of strain variations. The generation of heat in the wood components depends on the amplitude and frequency of the loading and on the structure, elasticity and internal friction of the wood [4].
The effect of frequency and amplitude of cyclic stress on fatigue in the mechanical breakdown of wood has been studied extensively [5, 6]. However, the frequency of cyclic loading has in all cases been much smaller (0.1 and 5 Hz) than that used in the production of mechanical pulps. To study fatigue in the wood fiber matrix the experimental parameters should match the physical conditions needed to break down the fiber structure. This imposes considerable demands on the testing equipment. Only when wood fatigue can be analyzed at high frequencies and ambient temperatures close to those in a mill grinder will we have the right tools for studying the physical behavior of wood under the conditions prevailing in mechanical pulping. OBJECTIVES The objective of this work was to introduce a new scientific tool for investigating the qualitative and quantitative responses of wood to mechanical loading at relevant conditions. The second objective was to investigate the response of wood to the impact of static load and cyclic stress pulses at variable amplitudes and frequencies. EXPERIMENTAL Modulated wood loading device A modulated wood loading device was designed and built at KCL. The device allows frequencies to be applied that are close to those used in production-scale grinders. Fatigue of the wood matrix is achieved considerably faster than with the devices used in previous studies [5]. Using the device it is possible to generate frequencies of 0-1000 Hz and simultaneously to load the wood with a static loading pressure in the range 0-7 bar. Wood samples of different sizes can be tested ranging from 90 mm x 100 mm x 10 mm up to 90 mm x 100 mm x 250 mm (width x depth x height). The principle of the loading device is shown in Fig. 1. Photos of the pulse generating actuator blades are shown in Fig. 2. The results reported in this paper were obtained under atmospheric conditions.
Figure 1. KCL wood loading device, which combines static pressure and cyclic loading features. The static pressure is generated by the shaft and the cyclic stress pulse is induced by a rotating percussion drilling machine-type vibrator. Two 7.5 kW AC motors drive the actuator rotor plate.
Figure 2. Rotor and stator plates of the mechanical actuator used in the KCL loading device.
Temperature measurements During cyclic loading, the temperature change inside the wood was measured using thermocouples and the distribution of wood surface temperature with a thermographic camera. The thermocouples used in the study were K-type (NiCr – Ni). They were inserted into the wood block through a drilled hole (1.5 mm) typically to a depth of approximately 40 mm, as seen in Fig. 3. Figure 4 shows a schematic picture of the experimental set-up. The thermographic camera was an Inframetrics SC 1000.
Figure 3. Experimental set-up for the KCL loading device with three thermocouples inserted into the wood sample.
Figure 4. Schematic picture of the experimental set-up.
DSC - Differential Scanning Calorimeter Thermoporosimetry was used to characterize the changes in wood microstructure generated by the modulated loading action. Tangential wood sections were cut with a microtome from the treated wood samples and untreated reference wood samples. Samples were taken from that part of the wood where the temperature rise during modulated loading was recorded. The earlywood and latewood parts of the annual rings were cut out and measured separately. The thermoporosimetry measurements were performed with a Mettler 821e Differential Scanning Calorimeter (DSC) at TKK Laboratory of Paper and Printing Technology. The thermoporosimetry technique is based on the melting point depression of an absorbate (in this case water) in the capillaries of porous materials [7, 8]. Melting point depression is related to pore size. The measurement reports total pore volume in the sample and the distribution of pore sizes between 1 and 216 nm. We concluded that an increase in pore volume is a sign of irreversible deformations in the wood fiber wall ultrastructure. RESULTS Effect of static loading pressure on temperature development For a porous material, such as wood, to absorb mechanical energy by cyclic loading a threshold loading pressure must be reached. As a consequence a superimposed cyclic stress pulse becomes effective. Figure 5 shows the temperature inside the wood for three static load pressures at a loading frequency of 333 Hz. An increase in the feed pressure promoted clearly the heat generation in the wood. The larger static stress on the wood promoted greater deformations and thus higher temperatures could be measured. One explanation could be that the wood under stress follows the movements of the vibrating surface more completely than unstressed wood, because of stronger springback due to the pre-stress in the material. At loading pressures below 5 bar very little or no formation of heat could be observed.
Static load 5- 7 bar
Actuator with amplitude of 0.5 mm
Wood sample
Solid wall
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Figure 5. Effect of static loading pressure on wood temperature. The size of the wood sample was 90 mm x100 mm x 100 mm (width x depth x height). Effect of loading frequency on temperature development It can be expected that, on a specific time scale, the higher the number of effective loading impacts the wood sample receives, the more heat will be generated. Figure 6 shows temperature development during cyclic loading when the static feed pressure was 6 bar and the loading frequency was 333 Hz and 500 Hz. The sample size in this set of trials was 90 mm x 100 mm x 20 mm (width x depth x height). The increase in temperature was faster at the higher (500 Hz) than at the lower (333 Hz) loading frequency. The reason for this is the greater number of pulses during the same time period. In order to get a more realistic idea of the amount of energy needed to cause structural changes in wood material, heat generation in the samples should be studied using the same number of stress pulses (Fig. 7). To reach the desired temperature of 140°C the number of pulses needed was 25 000 at the lower loading frequency (333 Hz) whereas a far greater number of pulses (35 000) was needed to reach the same temperature at 500 Hz. This is because at lower frequency wood has more time to follow the vibrating surface, which generates heat more efficiently as a greater amount of the amplitude of the vibrating actuator is used.
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Figure 6. Effect of loading frequency on temperature in spruce wood as a function of the loading time.
Figure 7. Effect of loading frequency on temperature in spruce wood as a function of the number of impacts.
Surface temperature distribution of wood samples during cyclic loading A thermographic camera was used to measure the surface temperature distribution of the wood sample. Figure 8 shows typical images obtained with a thermographic camera during cyclic loading. The dotted line in the figure shows the borders of the wood sample.
The temperature scale in the thermographic camera image is from 20°C to 50°C. The white color in the figure means that the temperature is over 50°C.
a) b)
Figure 8. Temperature profile of the wood sample after 30s a) and 60s b) of cyclic loading at a frequency of 333 Hz and static feed pressure of 6 bar. The size of the wood sample was 90 mm x 100 mm x 100 mm (width x depth x height). The arrows show the direction of loading.
Figure 9. Spruce cross section.
Generally, the temperature rise was initiated at the front where the cyclic loading was subjected and unexpectedly in the middle part of the wood. The farther we move from the stress front the more dampened is the strain and the lower is the energy dissipation. An interesting detail is that, the temperature in the middle of the wood sample was higher than in the other parts of the sample. A possible explanation for this could be differences in the chemical and structural composition of the wood in the pith, and also the fact that fiber rows lead in the radial direction towards the center of the wood (Fig. 9). Thus, the vibration propagates and converges at this point and thus generates heat more efficiently. Plastic deformations When a wood sample undergoes plastic deformation in a mechanical process, its properties change and it behaves differently from the original sample. It was assumed that if we repeated the cyclic loading treatment for a once-loaded wood sample, a difference in the formation of heat would be seen. The same wood sample was loaded on two consecutive days. Figure 10 shows the temperature of the fresh spruce sample during cyclic loading and Figure 11 the temperature of the same sample when it was loaded again after 20 hours. The fresh wood sample underwent a much sharper temperature increase (Fig. 10) than the wood sample which had been once cyclically loaded under high stress (Fig. 11). The increase in temperature in the latter case was smaller because the structure of the wood matrix had been already loosened and did not generate heat during cyclic loading. These figures indicate that irreversible plastic deformations occurred during cyclic loading.
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Figure 10. Temperature development of a fresh wood sample during cyclic loading.
Figure 11. Temperature development of the same wood sample during cyclic loading 20 hours later.
Thermoporosimetry measurements of the wood samples also showed that plastic deformation took place during modulated loading. Figure 12 shows that the cumulative pore volume of both the earlywood and latewood samples was higher after cyclic loading than that of the equivalent reference samples. This implies that the increase in porosity is related to the permanent fatigue in the cell wall.
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a) b)
Figure 12. Cumulative pore volume before and after cyclic loading measured from earlywood and latewood of spruce, a) heartwood, b) sapwood. The bars show 95% confidence intervals for the average values. Response of heartwood and sapwood to cyclic loading Modulated loading tests were also carried out with sapwood and heartwood samples separately. Figure 13 shows that the temperature increase in the sapwood sample during cyclic loading was lower. This is probably due to the higher moisture content of sapwood.
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Figure 13. Temperature development for heartwood and sapwood during cyclic loading. The thermoporosimetry results in Figure 12 show that the change in cumulative pore volume after cyclic loading was about the same in both the earlywood and latewood parts of the heartwood sample (Fig. 12a). In the sapwood sample the change in pore volume due to cyclic loading was greater for earlywood than for latewood (Fig. 12b). CONCLUSIONS Deeper research into the mechanical pulping process requires the right tools to produce controlled changes in the wood and fiber material. The KCL modulated loading device fulfills these requirements. The device allows wood samples to be subjected to a wide range of static loads, loading frequencies and amplitudes. New evidence that supports the fatigue hypothesis was gained in this study. Permanent deformations were detected by two analytical methods: as a change in heat generation ability and as a change in wood microstructure (increase in pore volume).
It was shown that the static load pressure had a marked effect on the dissipation of energy into heat. This is because the static load pressure pre-stresses the wood, which induces springback forces and thus enables strain changes at amplitudes closer to that induced by the mechanical actuator. Also, the superimposed cyclic loading frequency had an additional impact on the increase in temperature, but to a lesser extent than that due to the static load. A lower cyclic loading frequency indicates more effective heat generation. The responses of heartwood and sapwood during fatigue treatment were quite different, the temperature increase in heartwood being greater due to cyclic loading. The higher sapwood water content explains some of the slower temperature rise observed in this study. References 1. Björkqvist T., Lucander M. Grinding surface with an energy-efficient profile. Mechanical pulps: added value for paper and board. 2001 International Mechanical Pulping Conference, Helsinki, Finland, 4-8 June 2001, vol. 2, pp. 373-380. 2. Lucander M., Björkvist T. New approach on the fundamental defibration mechanisms in wood grinding. International Mechanical Pulping Conference, IMPC 2005, Oslo, Norway, 7-9 June 2005, pp. 149-155. 3. Björkvist T, Tienari M., Lucander M. Simulation of fatigue related variables in wood grinding. 2007 International Mechanical Pulping Conference, Minneapolis, USA, 7-9 May 2007. 4. Höglund H., Tistad G. Energy uptake by wood in the mechanical pulping process. SPCI, EUCEPA, International Mechanical Pulping Conference, Stockholm, Sweden, 18-21 June 1973, pp. 3:1-3:26. 5. Salmen L. The effect of the frequency of a mechanical deformation on the fatigue of wood. JPPS 13(1987):1, pp. J23-J28. 6. Salmen N.L. Fellers C., Tigerström A. The effect of loading mode on the energy consumption during mechanical treatment of wood. TAPPI International Mechanical Pulping Conference, Washington, DC, 13-17 June 1983, pp. 109-114. 7. Maloney T. C., Paulapuro H. Thermoporosimetry of pulp fibers. The science of papermaking, 12th Fundamental research symposium, Oxford, UK, 17-21 Sept. 2001, vol. 2, pp. 897-926. 8. Kärenlampi P. P., Tynjälä P., Ström P. Molecular fatigue in cell walls (Extended Abstract). 2002 Progress in paper physics seminar, Syracuse, NY, USA, 8-13 Sept. 2002 edited by Keller D. S., Ramarao B. V., pp. 240-243.
Fatigue treatment of wood by high-frequency cyclic loading
Sari Panula-Ontto, Mikael Lucander, Tiina Pöhler, Erkki Saharinen, Tomas Björkqvist
Motivation
• To substantially cut down the energy required in mechanical defibration, basic material science must be understood and exploited
• To study irreversible changes e.g. fatigue in wood, the loading conditions must be performed at high frequencies and ambient temperatures.
– These prerequisites set high requirements on the testing equipment
Objectives
• Introduce a new scientific tool for investigating the qualitative and quantitative responses of wood to mechanical loading at relevant conditions.
• Investigate the response of wood to the impact of static load and cyclic stress pulses at variable amplitudes and frequencies.
Experimental set-up
Static load5- 7 bar
Actuator with amplitude of 0.5 mm
Wood sample
Solid wall
Trial equipment
Static loading pressure
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Loading frequency
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333 Hz
500 Hz
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500 Hz
333 Hz
Spruce 6 bar, 333 Hz
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Surface temperature distribution
Surface temperature distribution
Plastic deformations
Repeated fatigue treatment after 20 hours
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Fatigue treatment for fresh wood sample
Plastic deformations
Sapwood
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Earlywood ref
Latewood after cyclic loading
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Response of heartwood and sapwoodto cyclic loading
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Summary
• The KCL modulated loading device allows wood samples to be subjected to a wide range of static loads, cyclic loading frequencies and strain amplitudes.
• Permanent deformations were detected by two analytical methods:
– as a change in heat generation ability and
– as a change in wood microstructure (increase in pore volume).
• The static load pressure had a marked effect on the dissipation of energy into heat.
• The cyclic loading frequency had an impact on the increase in temperature, but to a lesser extent than that due to the static load.
– A lower cyclic loading frequency induces a more effective heat generation.
• The responses of heartwood and sapwood during fatigue treatment were quite different.
– Cyclic loading had a greater impact on the temperature increase in heartwood than in sapwood
